1. Field of the Invention
The present invention relates to the field of the regenerative treatment of porous strictures, notably in construction. More particularly, the invention relates to the treatment of porous strictures provided with reinforcements prone to oxidation, particularly in reinforced concrete.
2. Description of the Relevant Art
Concrete forms a medium which is massive, rigid and porous all at the same time. The number and size of the pores vary greatly from one concrete to another depending on the quality of the component materials used and the way in which the concrete is mixed. Concretes intended for constriction in a damp environment or in a salty environment may have their composition and method of use adjusted in an attempt to reduce the size and number of the pores. However, concretes of this kind are subject to movements of the interstitial water, notably as a result of capillary rise coming from the foundations, weathering or spray or mist. The circulation of water is accompanied by a movement of ions.
In reinforced concrete constructions, the steel reinforcements have the function of increasing the tensile strength of the material. Good conservation of the steel therefore ensures that these constructions are long-lasting. In concrete that is in good condition the pH is high, for example around 12, and the steels are spontaneously covered by a natural protection known as passivation. In fact, carbon dioxide from the environment, particularly from the atmosphere, reacts with the cement that makes Up the concrete, forming carbonates, thereby reducing the pH of the solution contained in the pores of the concrete. This phenomenon known as carbonation may cause corrosion of the steel embedded in the concrete. The anions, e.g. chlorides, likely to be found in large quantities close to the steel, modify the nature of the products formed at the interface between the concrete and the steels and prove to be detrimental to passivation. The steels may then corrode. This is also true of the sulphates which may originate from a polluted atmosphere or water and migrate close to the reinforcements. If the reinforcements are corroded, the service life of the structure may be shortened as a result.
Processes are known which set out to regenerate reinforced concrete by re-alkalizing the medium around the reinforcements or by removing pollutants such as chlorides. Reference may be made to the European recommendations on electro chemical procedures for re-alkalizing carbonated concrete (CEN/TS14038-1) or extracting chlorides (pr/TS14038-2). This type of process uses an external electrode, the internal electrode being formed by the steel reinforcement. An electricity generator passes a current between the external electrode and the internal electrode, thus causing the anions to travel towards the surface of the concrete. The treatment is continued for a long time as a function of the concentration of pollutant.
However, the present Applicant has noticed that the electrical resistivity and capacitance of the concrete between the reinforcements and the surface electrode increase greatly. It has been discovered that this phenomenon is due to a steady state of movement of the chemical species in the pores of the concrete and close to the surface of the steels. One of the causes is the polarization which is liable to break down the water around the steels and form hydroxyl anions (OH′). At the same time, cations such as calcium may move towards the steels and form a precipitate with a high electrical resistance. The substantial increase in electrical resistance, by a factor of possibly more than 5, correspondingly reduces the polarization current if the electricity generator supplies a set voltage between the electrode and the reinforcements. If the generator is of the kind with a constant set current, the voltage will be greatly increased as a result and may exceed the maximum value allowed by safety regulations. Moreover, the energy consumption becomes very great. Another reason is in the variation in the composition of the liquid in the pores of the concrete which may temporarily alter the electrical charges on the walls of the pores until a new electrical equilibrium is achieved. If the system is shut down when the electrical resistance becomes too great, the time taken for the steel to depolarize naturally is very long, at least several months and possibly even more than a year.
It is also possible to apply a reverse voltage for a short period in an attempt to remedy the phenomena that cause the increase in electrical resistance and capacitance. In fact, having conducted intensive research into these techniques, the Applicant has discovered that during the circulation phase of a reverse current, hydrogen gas may be released at the external electrode. The presence of hydrogen gas is dangerous. The electrical resistance of the external electrode may also be increased as a result. In other words, the application of a reverse current setting out to lower the electrical resistance of the concrete produces an increase in the electrical resistance in the external electrode which also counteracts the circulation of the current and furthermore risks causing the electrode to heat up, with possible risks of fire or drying out of the electrode in the case of a wet electrode.
The present invention sets out to remedy the situation.